OBJECTIVE: Determination of bone strength of lower extremities in very low birth weight (VLBW) premature infants with central nervous system pathology resulting in reduced unilateral spontaneous leg movements. STUDY DESIGN: Quantitative ultrasound (QUS) measurements of speed of sound (SOS) of the tibiae of both legs in three VLBW premature infants with brain insult and unilateral reduced spontaneous activity. Results were compared to QUS measurements of both legs in healthy premature infants. Measurements were performed by the same investigator who was blinded to the clinical course of the participants. RESULTS: Reduced spontaneous activity of one leg due to brain pathology resulted in decreased tibial SOS in the affected side. There was no difference in bone SOS between the legs of the healthy controls. CONCLUSION: Spontaneous movements (mainly antigravity flexion and extension) are important for bone structure and mineralization in VLBW premature infants. QUS may become an important diagnostic modality for the evaluation, treatment, and follow-up of bone strength and osteopenia in this unique population.